1. Technical Field
This invention relates to a process for producing a cold rolled Al-killed steel sheet having an ageing resistance, which is equivalent to that obtained by box annealing, by continuous annealing including rapid heating and rapid quenching, particularly by utilizing a specific heat cycle for operations from the rapid quenching to the subsequent overageing. The term "sheet" means "sheet" or "plate" in the present specification and claims.
2. Description of the Prior Art
Box annealing has been so far an ordinary procedure for annealing a cold rolled steel sheet for working, but recently the cold rolled steel sheets are often produced by continuous annealing owing to remarkable quality and economical merits of the continuous annealing.
However, the continuous annealing has such a serious disadvantage that no satisfactory ageing resistance can be given to Al-killed steel, and thus the remarkable quality and economical merits of the continuous annealing have not been fully obtained in the case of the Al-killed steel up to now.
Several attempts have been so far made to improve the ageing resistance of Al-killed steel. For example, it has been proposed to improve the ageing resistance by devising a heat cycle for operations from quenching down to overageing by the following prior arts: Japanese Patent Publication No. 58-10447 disclosing a horizontal overageing process, in which the steel sheet reheated after the supercooling is retained at the same temperature in an operation after the reheating and the relationship between the temperature and time in such an operation of retaining for overageing is expressed by a horizontal linear line; and the Japanese Patent Publication No. 58-39890 and Japanese Patent Application Kokai (Laid-Open) Nos. 60-52527 and 61-276935 disclosing an overageing process according to an inclinatory cooling, in which the temperature of the steel sheet reheated after the super-cooling is changed with the passage of time in an operation after the reheating or the subsequent retaining and the relationship between the temperature and time in such an operation of cooling for overageing is expressed by an inclinatory line. However, the proposed processes fail to economically produce a cold rolled steel sheet having a good ageing resistance, because the heat cycle concept as well as the heat cycle itself has defects.